Ultrasonic sensors for motor vehicles are already known from the prior art in various designs. They are commonly employed for assisting the driver in maneuvering the motor vehicle, in particular in performing parking operations. By means of the ultrasonic sensors in this connection distances from obstacles are measured, which are present in the environment of the motor vehicle. The ultrasonic sensors here are part of a driver assistance device, which is referred to as parking aid. However, ultrasonic sensors today more and more frequently are employed outside this actual parking aid functionality, such as for instance for driving assistance with active braking intervention, i.e. with automatic braking assistance systems, in systems for blind spot monitoring, in systems for keeping a distance, collision recognition systems, and the like.
Ultrasonic sensors work according to the echo transit time principle: The distance measurement in ultrasound technology is performed by means of an echo transit time method or echo sounding method. The ultrasonic sensor emits an emission signal—ultrasound—and receives a receive signal, which equally is a sound signal and corresponds to a signal portion of the emission signal, which is emitted and reflected by an obstacle. This means that ultrasonic waves are emitted, reflected by an object, and received again and analyzed by the same ultrasonic sensor and/or an adjacent ultrasonic sensor of the same motor vehicle. In dependence on the measured transit time of the ultrasonic wave then the distance and possibly also the relative position and/or a relative speed relative to the motor vehicle are determined.
It may also occur that an ultrasonic sensor of a motor vehicle is blocked. Methods, which serve for recognizing a blocked state of an ultrasonic sensor, are already known from the prior art. In the prior art in this connection a situation is recognized, in which the ultrasonic sensor is covered by an additional mass, such as dirt and/or snow and/or ice. Since ultrasonic sensors nowadays more and more frequently are also used outside the actual parking aid functionality, in the operation of the motor vehicle it should be made sure that the ultrasonic sensors present on the motor vehicle reliably recognize the obstacles present in the environment of the motor vehicle respectively are also capable of reliably determining the distances up to a predetermined range. If the ultrasonic sensors are covered by an additional mass, this should be reliably detected. The known methods for recognition of ice or dirt are generally based on the analysis of side effects, which are caused by the additional mass on the ultrasonic sensor. Thus, by an additional mass for instance the so-called die-away time of the membrane of the ultrasonic sensor is influenced or a virtual echo or a fraudulent echo is generated, which can be detected by corresponding analysis of the electric receipt signal of the ultrasonic sensor.
In the subject matter according to DE 102 47 971 A1 the natural frequency or resonant frequency of the ultrasonic sensor is measured and compared with the stored reference values. This method is based on the fact that the resonant frequency of the ultrasonic sensor is a direct indicator of a soiling, ice or snow layer, since this additional layer affects the mass of harmonious vibration. With the additional mass of the dirt or the ice or snow layer namely also the vibrating mass and consequently the resonant frequency of the sensor changes.
In order to be able to recognize a state of an ultrasonic sensor covered in dirt and/or ice and/or snow, the DE 10 2009 040 992 A1 suggests, to determine the die-away time of the ultrasonic sensor subsequent to an excitation of the membrane and to compare it with the excitation frequency. Depending on the result of this comparison it is determined whether the ultrasonic sensor is blocked or not.
Further the DE 10 2010 021 960 A1 describes a method, in which for recognizing the blocked state of an ultrasonic sensor the die-away time of the membrane is analyzed over several measurement cycles of the ultrasonic sensor. A further plausibility check here can consist in that a temperature of the environment of the motor vehicle remains below a predetermined limit value. This limit value can for instance be 0° C. Thus the error rate in recognizing the blocked state can be reduced.
It has also turned out that whilst in some situations the ultrasonic sensor may be blocked, however this “blindness” of the ultrasonic sensor is not caused by an additional mass on the ultrasonic sensor itself. This is because situations have been detected, in which a “blindness” of the ultrasonic sensor has occurred even without an essential change of the die-away time of the membrane as well as without change of the resonant frequency.
It is task of the invention in a method of the initially mentioned kind and a motor vehicle to take measures that allow for a reliable operation of the at least one ultrasonic sensor.
This task according to the invention is solved by a method and a motor vehicle having the features according to the respective independent patent claims.